1. Field of Invention
This invention relates to the protection of a CRT phosphor screen from damage caused by an impinging electron beam which is stationary, or which has a low sweep velocity, and more particularly to the high speed detection of low velocity horizontal and vertical sweep signals during the period of time in which a video signal is present at the CRT.
2. Description of the Prior Art
It is well known that the phosphor screen of a CRT is highly sensitive to electrons which impinge its surface in the form of a beam, and that the CRT is subject to the failure of having the phosphor coating damaged in the area contiguous to the point on which the electron beam strikes the coating if the electron beam is allowed to remain stationary at that location for a period of time, of from 10 to 100 microseconds, depending upon the degree of electron sensitivity of the coating, or if the beam traverses the screen at a low sweep velocity. Since a stationary, or low sweep velocity beam is due to a failure of the beams horizontal and vertical sweep positioning signals, numerous prior art protection circuits have been devised which monitor the sweep signals, or the intensity of the video signal, or both, and remove the electron beam when a failure is detected.
One such method is described in the patent to Henderson et al, U.S. Pat. No. 3,437,872, wherein a means for independently monitoring the velocity of the horizontal and vertical sweep signals, and a separate means for monitoring the intensity of the video signal are used to detect independent failures of these three parameters, such that any one failure is sufficient to remove the video signal from the CRT. This method has severe disadvantages in CRT displays incorporating the stroke-write method of displaying alpha-numeric characters, wherein each character is displayed by displaying series of stroke vectors separated by interim wait periods at the completion of each vector during which the beam sweep signals are stationary and the beam is allowed to settle out, and by periods in which the video signal is removed while the beam is repositioned to a new location on the face of the CRT. The interim periods in which the beam sweep signals are stationary would be detected as failures in this protection circuit unless the means used for monitoring the sweep signals is designed to disregard these periods, such as by delaying the detection of the presence of the sweep signals. The magnitude of the time delay must be larger than that of the worst case summation of all successive time periods in which the sweep signals may be stationary in normal operation. In a stroke-write display system, the time delay would therefore have to account for the beam deflection time, wherein the beam is changing angular position, the settling time required after the beam is repositioned to a new location, and the period of time required for writing periods at the end of a sentence of text: this total time is typically in the order of 50 microseconds, which far exceeds the minimum ten microseconds time that may be tolerated before damage occurs in high sensitivity phosphor coatings. Therefore, this protection circuit would be inadequate to protect a high sensitivity phosphor coating in a stroke-write vector display system.
In a patent to R. G. Curry, Jr., U.S. Pat. No. 3,811,068, a phosphor protection circuit, which monitors the presence of sweep signals, is implemented with logic circuitry. Therein, the protection circuit essentially compares the presence of a sweep voltage signal with a coincident presence of an unblank signal (the presence of an unblank signal allows the video signal to be displayed on the face of a CRT), and the failure of the sweep signal during the period in which an unblank signal is present is used to trigger a bistable latching means whose output is received by a suitable blanking circuit, not disclosed. This method eliminates the need for the extended analog time constant as described hereinbefore, however a failure cannot be detected in less time than that equivalent to one sweep period, which depending upon the frequency of the display may be protracted and therefore insufficient to ensure protection within the minimum ten microsecond period.
A patent to J. H. Meacham, U.S. Pat. No. 3,810,024, describes another protection circuit using logic circuitry which again compares the presence of sweep signals with the simultaneous non-appearance of a blanking signal (unblank signal), to detect failures of the sweeping circuit. The improvement described therein relates to the ability of the protection circuit to detect failures and remove the video signal within a time equivalent to one-half of the sweep period, rather than the full sweep period time required by the prior art. This however, still results in a dependency upon the frequency of the sweep signals, and the failure detection may not occur for an extended period of time.
In summary, the prior art protection circuits do not provide adequate protection against burn out of high sensitivity phosphor coatings, such as those used in stroke-write vector display systems.